1. Field of the Invention
The present invention relates generally to an automatic frequency response plotting system and, more particularly, to improvements in circuitry in such a system.
2. Description of the Prior Art
Frequency response plotting systems are presently available which are capable of producing a plot of the frequency response of a device under test, e.g., a speaker, amplifier or filtering network over a selected frequency range. The plot is produced by an XY-plotter which includes a pen which is mechanically movable along the X-axis, along which frequency is represented on the plot, as well as along the Y-axis along which the response of the device under test is plotted as a function of frequency. Typically, the system circuitry includes a send section which includes a voltage ramp generator which generates a voltage which ramps from a first level to a second higher level at a controllable rate. This ramp voltage is used to control the pen movement along the X-axis of the plotter from one end of the desired plot to the other end. Also, the ramp voltage is used to activate a frequency oscillator which is swept in frequency over the range of interest, e.g., 20Hz to 20KHz, when plotting the response of a device operable in the audio range. The output of the oscillator is supplied to an amplifier whose output is constant over the frequency range of interest. It is this output which is applied to the device under test. The ramp voltage activates the oscillator so that at each position of the plotter pen along the X-axis the oscillator frequency corresponds to the frequency designated on the plot at the particular X-axis position of the pen.
The circuitry also includes a receiver section which responds to the output of the device under test to which the constant amplitude variable frequency signal is applied as an input from the send section output amplifier. The receiver section includes means which rectify the AC output of the device under test and uses the rectified output to control the pen position along the Y-axis of the plotter.
The mechanism used to move the pen along the X and Y axes are mechanical in nature and therefore of limited response. Consequently, whenever the device under test is known or suspected of exhibiting significantly large changes in response as a function of small frequency variations, which would result in steep amplitude excursions along the Y-axis, a very low sweep rate must be employed to enable the pen driving mechanism to respond to the large changes in the device's output so as to move the pen in the Y-axis to produce the steep amplitude excursions without distortion.
In the prior art the sweep rate is not changed during the response plotting as a function of changes in the rate of change of the device's output. If the device to be tested is known or suspected of having a complex response pattern, which would result in steep amplitude excursions in the plot over the entire frequency range, or over any portion or portions thereof, a very slow sweep rate is recommended, which increases the plotting time. On the other hand, if the device under test is expected to have a relatively uniform response over the entire range of interest, i.e., its output is expected to vary gradually as a function of frequency change, a high sweep rate may be employed. If however, the device output exhibits unsuspected steep amplitude changes, such as sharp dips or peaks, and the system is operating at a high sweep rate, because of the limited ability of the plotter mechanism to respond to the steep amplitude changes, they are missed or greatly minimized, thus resulting in a distorted or inaccurate response plot.